The Japanese mouse, M.m molossinus, carries a locus apparently identical to the Friend virus resistance gene, Fv-4, described originally in G strain mice. Sensitivity to Friend SFFV disease is now being examined in these mice to characterize Fv-4 and identify any other genetic factors which influence virus replication and the development of disease. To date, this analysis has resulted in the identification of a second locus on chromosome 5 which influences sensitivity to Friend virus, and the description of a sexual bias in susceptibility to disease. Other mouse species are currently being examined for sensitivity to virally-induced disease. More recent experiments have described the in vitro restriction of virus replication by cells carrying Fv-4r. Genetic crosses are currently being tested to determine whether any of the novel MuLV reactive fragments identified by blot hybridization in Fv-4r mice are associated with resistance